Catalyst for preparing hydrocarbon

ABSTRACT

The present invention relates to a catalyst for preparing hydrocarbons of carbon dioxide and more particularly, the Fe—Cu—K/γ—Al 2 O 3  catalyst prepared by impregnation which enables producing hydrocarbons in high yield for more than 2000 hours due to its excellent activity and stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for preparing hydrocarbons by hydrogenation of carbon dioxide and more particularly, the Fe—Cu—K/γ—Al₂O catalyst prepared by impregnation which enables producing hydrocarbons in high yield for more than 2000 hours due to its excellent activity and stability.

2. Description of Related Art

Carbon dioxide is one of the so-called greenhouse gases. It is known that it absorbs infrared energy and prevents such energy from leaving the atmosphere. Accumulation of carbon dioxide in the atmosphere caused by a huge amount of the fossil and organic-based fuel consumption may therefore contribute to an increase in average global temperatures, resulting in more frequent and more violent natural catastrophes (e.g., periods of drought, flood, storm). However, the world's growing population and increasingly technological society have made it difficult for the world's energy and material resources to keep pace with the current trends. Therefore, increased efforts are required to conserve the environment by restricting the use of fossil and organic-based fuel. To activate this, recycling of carbon dioxide into useful hydrocarbons would help to alleviate the problem of our diminishing hydrocarbon resources.

In order to convert carbon dioxide to useful hydrocarbons, the inventors have disclosed a process for preparing hydrocarbons over a Fe—K/γ—Al₃O₃ catalyst performed by flowing gas mixture (H₂/CO₂=1.0˜5.0 v/v) into the reactor at 200˜500° C., 1˜100 atm and 500˜20,000 h⁻¹ of the space velocity (U.S. Pat. No. 5,952,540). A catalyst used in this process is pretreated Fe—K/γ—Al₂O₃ by reduction and activation, contains 5˜50 wt. % of Fe to total catalyst weight and includes 0.1˜1.5 of atomic ratio of K/Fe. However, when Fe—K/γ—Al₂O₃ catalyst has been used for more than 800 hours, the conversion rate of carbon dioxide to hydrocarbons is decreased by more than 10% and productivity of hydrocarbons is also decreased by more than 20%. On top of that, formation of undesirable carbon monoxide and methane gas are extremely increased by 70% and 30%, respectively.

SUMMARY OF THE INVENTION

In view of the above considerations, the inventors herein have intensively studied to solve deactivation of a catalyst for preparing hydrocarbons out of carbon dioxide by hydrogenation. As a result, the inventors realized that depositions of carbonaceous material such as coke on the surface of the catalyst is a major cause of deactivation of Fe—K/γ—Al₂O₃ catalyst as disclosed in U.S. Pat. No. 5,952,540 and thus, addition of Cu can provide excellent stability of the catalyst for more than 2000 hours to prepare hydrocarbons by suppressing depositions at carbonaceous on the surface of the catalyst.

Therefore, the object of the present invention is to find a catalyst which not only gives satisfactory activity values but at the same time provides long-term resistance to deactivation by depositions of carbonaceous. Thus, the Fe—K—Cu/γ—Al₂O₃ catalyst prepared by adding Cu to the conventional Fe—K/γ—Al₂O₃ catalyst provides that it decreases coke deposits thereon and exhibits sufficient activity, satisfactory yield of hydrocarbons even after using more than 2000 hours and good stability.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst of the present invention used for preparing hydrocarbons by hydrogenation of carbon dioxide is Fe—Cu—K/γ—Al₂O₃ including 5˜50 wt. % of Fe to total catalyst weight and containing 0.07˜1 of weight ratio of K/Fe and 0.05˜0.5 of weight ratio of Cu/Fe.

A process for preparing hydrocarbons by hydrogenation of carbon dioxide is performed by flowing gas mixture (H₂/CO₂=1.0˜5.0 v/v) into the reactor at 200˜500° C., 1˜100 atm and 500˜20,000 h⁻¹ of the space velocity over a catalyst, wherein the catalyst is a pretreated Fe—Cu—K/γ—Al₂O₃ by reduction and activation, including 5˜50 wt. % of Fe to total catalyst weight and containing 0.07˜1 of weight ratio of K/Fe and 0.05˜0.5 of weight ratio of Cu/Fe.

The detailed description of the present invention is given hereunder.

The present invention relates to the Fe—Cu—K/γ—Al₂O₃ catalyst which not only gives satisfactory activity values but at the same time provides resistance for more than 2000 hours to deactivation by precipitation of carbonaceous.

The catalyst used in the present invention includes copper(Cu) as well as iron(Fe) and potassium(K) supported on γ-alumina (γ—Al₂O₃) carrier, wherein a certain amount of copper(Cu) replaced in the active site of iron(Fe) can serve to reduce deposits of carbonaceous and thus, provide excellent activity and stability for long period. A process for preparing the catalyst is as follows.

Aqueous solution of iron-containing salts, potassium-containing salts and copper-containing salts is impregnated into aluminum oxide and mixed. Then the mixture was dried at 80˜200° C. for 5˜48 hours and calcinated at 400˜700° C. for 5˜48 hours. A catalyst obtained by the above process becomes mixed state of iron oxide, potassium oxide, copper oxide and aluminum oxide.

Said salts include hydrochloric acid salt, nitric acid salt, sulfuric acid salt, acetic acid salt, oxalic acid salt and various aqueous salts. For example, iron-containing salt is selected from the group consisting of iron chloride, iron nitrate, iron sulfate, iron acetate and iron oxalate. Potassium-containing salt is selected from the group consisting of potassium chloride, potassium carbonate, potassium nitrate and potassium acetate. Copper-containing salt is selected from the group consisting of copper chloride, copper nitrate and copper sulfate.

It is preferred to contain 5˜50 wt. % of Fe to total catalyst weight, 0.07˜1 of weight ratio of K/Fe and 0.05˜0.5 of weight ratio of Cu/Fe, and to use γ—Al₂O as a carrier.

According to the present invention to add copper as an activity promoter, if the weight ratio of copper to iron is less than 0.05, it is not sufficient to avoid deactivation by deposits of carbonaceous. If the weight ratio of copper to iron is more than 0.5, the activity of a catalyst becomes extremely poor due to the insufficient iron-surface area as an activity promoter.

Conventional catalysts containing Fe, Cu, K and Al have been reported but the weight ratio of potassium to iron was not more than 0.07 and aluminum oxide was used as a simple carrier without any limitation. However, 0.07˜1 of weight ratio of K/Fe increase and use of γ—Al₂O₃ in the present invention are essential to obtain excellent activity of a catalyst. γ—Al₂O₃ not only plays a role of a carrier but also increases activity and selectivity of a catalyst through interaction with iron. If the weight ratio of potassium to iron is less than 0.07, it is not proper to activate carbon dioxide to convert to hydrocarbons because alkalinity of a catalyst is not sufficiently increased. In case of using silica which only plays a role of a carrier, activity and selectivity of iron-based catalyst were not increased. Where γ—Al₂O₃ is used, if the content of supported iron is less than 5 wt. %, activity of a catalyst becomes poor and if it is more than 50 wt. %, proper interaction of γ—Al₂O₃ with aluminum oxide cannot be gained.

According to the present invention to obtain sufficient activity, Fe—K—Cu/γ—Al₂O₃ catalyst must be pretreated by reduction and activation. Reduction is performed by flowing hydrogen on Fe—Cu—K/γ—Al₂O₃ at 300˜500° C., 1˜10 atm and 20˜100 ml/g-cat./min of the flow rate, iron oxide reduced changes to iron metal. Activation is performed by flowing the gas mixture (H₂/CO₂=0.2˜10 v/v) on Fe—Cu—K/γ—Al₂O₃ at 200˜400° C., at 10˜40 atm and 2˜200 ml/g-cat./min of the flow rate and then flowing a gas selected from the group consisting of nitrogen, argon and helium on Fe—Cu—K/γ—Al₂O₃ at 100˜400° C., 1˜10 atm and 10˜100 ml/g-cat./min of the flow rate. Iron of the activated Fe—Cu—K/γ—Al₂O₃ catalyst exists as an appropriate carburized state, and this effectively acts on activating of carbon dioxide.

The reactor for preparing hydrocarbons from carbon dioxide can be a fixed bed reactor, a fluidized bed reactor or a slurry type reactor of liquid phase. The hydrogenation of carbon dioxide is performed under a gas mixture (H₂/CO₂=1.0˜5.0 v/v) at 200˜500° C., 1˜100 atm and 500˜20,000 h⁻¹ of the space velocity. Stoichmetric ratio of the gas mixture (H₂/CO₂) is 3˜4 v/v. If the condition of hydrogenation is out of this range, the good conversion of carbon dioxide cannot be expected. For example, if the ratio of the gas mixture is less than 1.0 v/v or more than 5.0 v/v, the conversion of carbon dioxide becomes too low. If reaction temperature is lower than 200° C., the conversion rate goes down, and if it is higher than 500° C., the selectivity to C₂₊ hydrocarbons become low because the higher reaction temperature is, the more formation of methane is. If reaction pressure is less than 1 atm, reaction rate is too slow and if it is more than 100 atm, it is difficult to control the reaction. If the space velocity is less than 500 h⁻¹, the selectivity is too low and if it is more than 20,000 h⁻¹, the conversion is low because the contact time between reactant and catalyst is too short.

Fe—Cu—K/γ—Al₂O₃ catalyst has the best activity from 5 hours to 1,000 hours after the reaction starts. Even though the activity of catalyst is slowly getting decreased after 1000 hours it still provides relatively stable activity due to small different between the conversion amount and the formation amount of hydrocarbons.

According to the above description Fe—Cu—K/γ—Al₂O₃ catalyst pretreated by reduction and activation process is used to generate hydrocarbons from carbon dioxide by hydrogenation for more than 2000 hours in much higher yield and conversion rates than those of processes over conventional catalysts.

This invention may be illustrated in more detail by the following examples but it is not limited by the examples.

EXAMPLE 1

To 36 g of Fe(NO₃)₃·9H₂O, 1.25 g of Cu(NO₃)₂·3H₂O and 3 g of K₂CO₃ dissolved in 100 g of water was added 20 g of γ—Al₂O₃. The mixture was vigorously stirred and heated to evaporate water. After the evaporation of water, the reaction mixture was dried at 120° C. for 24 hours and calcinated at 450° C. for 6 hours. To reduce 0.5 g of calcination-completed Fe—Cu—K/γ—Al₂O₃ catalyst in a flow-type reactor hydrogen was flowed at 450° C. and 60 ml/g-cat./min for 24 hours. To activate the catalyst the mixture of carbon dioxide and hydrogen (H₂/CO₂=3 v/v) was flowed at 300° C., 10 atm and 32 ml/g-cat./min for 16 hours, and then nitrogen was flowed at 200° C., 1 atm and 20 ml/g-cat./min for 1 hour.

The mixture gas of carbon dioxide and hydrogen (H₂/CO₂=3 v/v) was flowed into the pretreated Fe—Cu—K/γ—Al₂O₃ catalyst at 300° C., 10 atm and 1800 h⁻¹ of the space velocity to give hydrocarbons.

The results are given in the following table. 1.

EXAMPLE 2

To 36 g of Fe(NO₃)₃·9H₂O, 1.5 g of Cu(NO₃)₂·3H₂O and 0.9 g of K₂CO dissolved in 100 g of water was added 20 g of γ—Al₂O₃. The mixture was vigorously stirred and heated to evaporate water. After the evaporation of water, the reaction mixture was dried at 120° C. for 24 hours and calcinated at 450° C. for 6 hours. To reduce 0.5 g of calcination-completed Fe—Cu—K/γ—Al₂O₃ catalyst in a flow-type reactor hydrogen was flowed at 450° C. and 60 ml/g-cat./min for 24 hours. To activate the catalyst the mixture of carbon dioxide and hydrogen (H₂CO₂=3 v/v) was flowed at 300° C., 10 atm and 32 ml/g-cat./min for 16 hours, and then nitrogen was flowed at 200° C., 1 atm and 20 ml/g-cat./min for 1 hour.

The mixture gas of carbon dioxide and hydrogen (H₂/CO₂=3 v/v) was flowed into the pretreated Fe—Cu—K/γ—Al₂O₃ catalyst at 300° C., 10 atm and 1800 h⁻¹ of the space velocity for 6 hours to give hydrocarbons.

The results are given in the following table 1.

COMPARATIVE EXAMPLE 1

The catalyst was prepared by the procedure of Example 1 without using Cu(NO₃)₂·3H₂O instead of 1.25 g of Cu(NO₃)₂·3H₂O.

The results are given in the following table 1.

COMPARATIVE EXAMPLE 2

The catalyst was prepared by the procedure of Example 1 by using 0.8 g Cu(NO₃)₂·3H₂O instead of 1.25 g of Cu(NO₃)₂·3H₂O.

The results of activity after 100 hours are given in the following table 1.

COMPARATIVE EXAMPLE 3

The catalyst was prepared by the procedure of Example 1 by using 9 g of Cu(NO₃)₂·3H₂O instead of 1.25 g of Cu(NO₃)₂·3H₂O.

The results of activity after 100 hours are given in the following table 1.

COMPARATIVE EXAMPLE 4

The catalyst was prepared by the procedure of Example 1 by using silica (SiO₂) instead of γ-alumina (Al₂O₃).

The results of activity after 100 hours are given in the following table 1.

COMPARATIVE EXAMPLE 5

The catalyst was prepared by the procedure of Example 1 by using 0.3 g of K₂CO₃ instead of 3 g of K₂CO₃.

The results of activity after 100 hours are given in the following table 1.

COMPARATIVE EXAMPLE 6

The catalyst was prepared by the procedure of Example 1 excluding activation step of flowing the mixture gas of carbon dioxide and hydrogen.

The results of activity after 100 hours are given in the following table 1.

TABLE 1 Con- Reaction version Yield (C %) Examples time rate (%) Hydrocarbons C₁ C₂₊ Example 1 6 40.1 36.4 3.6 32.8 100 40.9 37.3 5.7 31.6 1000 40.5 37.0 5.5 31.5 2005 38.3 34.6 5.9 28.7 Example 2 6 33.6 27.3 9.2 18.1 100 32.6 25.9 9.2 16.7 1000 32.5 25.6 8.2 17.4 2005 30.7 23.4 8.4 15.0 Comp. Example 1 100 31.3 24.3 2.6 21.7 850 26.4 16.0 2.3 13.7 Comp. Example 2 100 32.8 25.6 3.5 22.1 Comp. Example 3 100 20.8 8.5 5.1 3.4 Comp. Example 4 100 15.7 6.3 0.9 5.4 Comp. Example 5 100 31.5 25.2 10.2 15.0 Comp. Example 6 100 29.7 23.0 2.3 20.7

As shown in table 1, the Fe—Cu—K/γ—Al₂O₃ catalyst keeps its activity even after 2000 hours to produce hydrocarbons from carbon dioxide in high yield. 

1. A process for preparing hydrocarbons by hydrogenation of carbon dioxide performed by flowing a gas mixture (H₂/CO₂=1.0˜5.0 v/v) into the reactor at 200˜500° C., 1˜100 atm and 500˜20,000 h⁻¹ of the space velocity over a catalyst, wherein said catalyst is a pretreated Fe—Cu—K/γ—A1₂O₃ by reduction and activation, including 5˜50 wt. % of Fe to total catalyst weight and containing 0.07˜1 of weight ratio of K/Fe and 0.05˜0.5 of weight ratio of Cu/Fe, wherein said activation is performed by flowing a gas mixture (H₂CO₂=0.2˜10 v/v) on Fe—Cu—K/γ—Al₂O₃ at 200˜400° C., 10˜40 atm and 2˜200 ml/g-cat./min of the flow rate and then flowing a gas selected from the group consisting of nitrogen, argon, and helium on said Fe—Cu—K/γ—Al₂O₃ at 100˜400° C., 1˜10 atm, and 10˜100 ml/g-cat./min of the flow rate.
 2. The process according to claim 1, wherein said reduction is performed by flowing hydrogen on said Fe—Cu—K/γ—Al₂O₃ at 300˜500° C., 1˜10 atm, and 20˜100 ml/g-cat./min of the flow rate. 